The present invention relates to drill bits.
The present invention more particularly relates to drill bits for drilling holes in concrete and asphalt.
Holes can be drilled in concrete and asphalt, using pneumatically powered drilling machines. Such drilling machines commonly include mechanisms for rotating the drill bit, and simultaneously vibrating the drill bit axially. The cutting action may be considered as a combined axial percussive fracturing of the concrete surface, and a rotary scraping of the surface for dislodging the concrete particles loosened by the fracturing process. The drilling machines may be handoperated portable machines, or machines supported on trailers or other wheeled vehicles.
The drill bits commonly are elongated steel shafts having hexagonal cross sections. The terminal end of the hexagonal shaft carries a hardened drilling insert, extending transversely thereacross; the cutting insert usually has a sharpened V-shaped cutting edge formed of tungsten carbide. As the drill bit rotates and vibrates axially, the sharpened V-shaped cutting edge delivers percussive blows to the concrete, thereby fracturing the concrete surface.
Concrete fragments produced by the drill bit are commonly removed from the drilled hole by feeding pressurized air downwardly through a passage system in the drill bit. The pressurized air is injected into the drilled hole in near proximity to the cutting edge of the drilling insert, whereby the pressurized air flushes the concrete debris upwardly, out of the drilled hole, and through the segmental spaces formed between the hole side surface and the flat side surfaces of the hexagonal bit. The pressurized air also serves the useful function of cooling the drilling insert, thereby somewhat prolonging the drill bit service life, before the insert has to be resharpened (or eventually discarded).
Pneumatically operated drilling machines usable in concrete are available from multiple sources, e.g. Chicago Pneumatic, Harper, Jet and Sullair. Usable drill bits are available from multiple sources, e.g. Bicknell Manufacturing Co., in Rockland, Me.
In recent years it has become necessary to drill holes in concrete roadways having an asphalt overlay, that defines the road surface. Such drilled holes are used by gas utility companies for installation of sniffer tubes in the concrete, for the purpose of monitoring, or testing, for gas leaks. It has been found that when conventional pneumatic drilling machines and drill bits are used to drill holes in concrete overlaid with asphalt, the drill bits tend to become "seized" in the asphalt. During the drilling operation, the smooth steel sides of the hexagonal drill bit become heated due to frictional contact with the asphalt, the concrete, and particulates generated by the drilling process. The asphalt partially melts and becomes sticky, causing at least a partial seal to form between the side surfaces of the drill and the asphalt. This partial seal causes a blockage of the air flow, and an ensuing build up of heat at the drilling insert. In some cases the drill bit will break off due to the heat build-up.
As a related matter, when the drill bit becomes "seized" by the warmed asphalt, the human operator of the drilling machine will often try to force the drill bit into, or out of, the drilled hole in an effort to complete the hole, or free the bit from its seized condition. In some cases the operators have experienced lower back pain and soft tissue damage, and some Workers Compensation claims have been filed as a result of such damage or injury.